(1) Field of the Invention
This invention relates to a gamma corrected LCD panel, and more particularly to an LCD panel having its RGB gamma curves adjusted to a preset white-balance point. Also, the invention relates to a method for producing the aforesaid panel.
(2) Description of Related Art
Recently, liquid crystal displays (LCD) become more popular than ever as a displaying component used in personal digital assistants (PDA), notebooks (NB), digital cameras (DC), digital videos (DV), mobile phones, etc. Due to lack of self-illumination, a cold cathode fluorescent lamp (CCFL) is required in an LCD as a backlight source. In addition, the LCD also needs a liquid crystal (LC) driving circuit to decode the input signals into corresponding LC operation voltage levels so as to adjust the transparency of RGB pixels for achieving full-color imaging.
FIG. 1 depicts a typical relationship of pixel devices' transparency and the applied electric field. In FIG. 1, the horizontal axis is the applied electric field strength with a unit of voltage (V), and the vertical axis is the relative transparency. Three data curves referred respectively as R (“red”), G (“green”), and B (“blue”) represent testing results of the pixel devices with particular displaying colors. As shown, the separation between any two curves implies that the LC layer in the pixel devices present different refractivity and different retardation value in response to different wavelengths of the passing-through visible light beams.
In order to quantify a look-up feeling of human eyes upon the LCD, gamma curves as typically shown in FIG. 2 are usually used to symbolize relationships between relative transparency and the bit numbers of the pixel devices. By having the transparency as a comparison basis between FIG. 1 and FIG. 2, a correlation between the bit number and the applied electric field can be established, and thereby the LC driving circuit can be designed much easier. However, the RGB gamma curves of FIG. 2 are separated to each other, and therefore it would be difficult to keep the combination of R, G, and B illuminations at a preset white-balance point. Also, bias in displaying color will definitely exist in response to the inputted displaying signals.
The arts to solve the separation problem in gamma curves as shown in FIG. 2 can be divided into two categories. One is to utilize electric circuit controlling means, and the other is to utilize structural adjusting means. In the latter category, efforts are provided to have the RGB pixel devices operating at LC layers with various thicknesses. FIG. 3 depicts a schematic cross-section view of such RGB pixel devices in accordance with the method targeting on varying LC layer thickness. As shown, each pixel device comprises an upper substrate 100, a lower substrate 300, and an interposed LC layer 200. Color filter layers 110R, 110G, 110B are formed on a lower surface of the upper substrate 100 to let the pixel devices show a preset color. Transparency organic layers 312R, 312G, 312B are formed on an upper surface of the lower substrate 300. Pairing of a pixel electrode layer 310 formed on the transparency organic layers 312 and a common electrode layer 120 under the color filter layers 110 creates an electric field E to drive the LC layer 200. Furthermore, two alignment films 130 and 320 are formed respectively on the inner surfaces of the common electrode layer 120 and the pixel electrode layer 310 to decide the orientation of the LC layer 200. As shown, the RGB pixel devices assign various thicknesses to the transparency organic layers 312 and the color filter layers 110, so as to vary local thickness of the LC layer 200.
It is well understood that the thickness of the LC layer 200, a spacing between the pixel electrode layer 310 and the common electrode layer 120, can severely affect the strength of the formed electric field E. Also, it is clear that the thickness of the LC layer 200 and the strength of the electric field E are both related to the transparency of the LC layer 200 and also the pixel device. Therefore, by assigning different thickness to the color filters 110R, 110G, 111B and the transparency organic layers 312R, 312G, 312B, the transparency of the LC layer 200 can then be adjusted and thereby the RGB gamma curves shown in FIG. 1 can have better coherence.
However, the above-described gamma adjusting method has the following drawbacks.
1. For additional steps of forming the transparency organic layers 312 with different thickness are needed before the step of forming the pixel electrode layer 310, so the fabrication cost will definitely increase.
2. The transparency organic layers 312R, 312G, 312B with various thicknesses can form respectively rough upper surfaces, on which the alignment film 320 is hard to form.
3. The pixel electrode layer 310 is known to be formed on the transparency organic layers 312 and thus it is lay on a rough surface formed by the aforesaid various thicknesses. Therefore, a lateral electric field may exist in the LC layer 200 to disturb normal operation of the pixel devices.
Accordingly, an improved gamma correction method in structure changes to avoid the above drawbacks is definitely welcome to the skilled in the art.